As a conventional ultrasonic probe, as shown in FIG. 8, a plurality of piezoelectric elements 21 for transmitting and receiving ultrasonic waves are arrayed in a direction orthogonal to the drawing, and a ground electrode 2 and a signal electrode 22 are placed on the front surface and the rear surface of each of the piezoelectric elements 21, respectively (for example, refer to the following patent document 1). An acoustic matching layer 24 for efficiently transmitting and receiving the ultrasonic waves to and from a subject (living body) is formed on the front surface of the ground electrode 2. A backing load member 25, which has a function for attenuating the unnecessary ultrasonic waves transmitted from the piezoelectric element 21 and mechanically holding the piezoelectric element 21 through the signal electrode 22 is placed on the rear surface of the piezoelectric element 21 on the side opposite to the acoustic matching layer 24. The ground electrode 2 is linked to a grounded electric terminal 23, and the grounded electric terminal 23 is configured to be connected through a heat conduction member 26 to a heat conduction line 27.
In this ultrasonic probe, the main body of the ultrasonic diagnostic apparatus or the like applies an electric signal through a signal electrode (not shown) and the grounded electric terminal 23 to the respective signal electrodes 22 and ground electrodes 2. Consequently, the piezoelectric elements 21 are mechanically vibrated to transmit the ultrasonic waves, and the ultrasonic waves reflected from the subject such as the living body are received by the piezoelectric elements 21. The ultrasonic probe for the ultrasonic diagnostic apparatus that uses the living body as the subject is the sensor used for the diagnosis where it directly contacts with the inside of the living body and transmits the ultrasonic waves to the living body, and reflection waves reflected from the inside of the living body is again received by the ultrasonic probe, and its signal is processed by the main body, and a diagnostic image is displayed on a monitor.
In such a way that the ultrasonic probe for the ultrasonic diagnostic apparatus as mentioned above has no bad influence on the living body, the surface temperature of the ultrasonic probe in contact with the living body must be kept at the temperature which has no influence on the living body. As for the surface temperature of the ultrasonic probe, in situation that it does not contact with the living body, namely, in unused situation, and in situation that a transmission signal continues to be sent from the main body, heat is generated to raise the temperature. This main reason is assumed to result from the dielectric loss of the piezoelectric element 21 and the multiple reflection between the piezoelectric element 21, the acoustic matching layer 24 and an acoustic lens inside the probe. In this way, the surface temperature of the ultrasonic probe has the relation proportional to the transmission signal of the main body, and the limitation is actually executed such that the transmission signal is suppressed to the small value and adjusted so as not to raise the temperature. On the other hand, the transmission signal level and the depth at which the living body is diagnosed have the proportional relation. The suppression of the transmission signal to the small value results in the demerit that the diagnostic depth is also shallow. Thus, it is very important to make the transmission signal higher (make the diagnostic depth deeper) and also make the surface temperature of the ultrasonic probe lower.
In this way, the ultrasonic probe used in the ultrasonic diagnostic apparatus is in direct or indirect contact with the living body. Thus, in order to insure the safety, the surface temperature of the probe is regulated and must be managed. For this reason, by adjusting a voltage transmitted from the ultrasonic diagnostic apparatus body, it is set low and managed such that it is below the regulation temperature. On the other hand, there is the strong request for desiring to enlarge the diagnostic region of the ultrasonic diagnostic apparatus, especially, the depth direction. The foregoing transmission voltage and the enlargement of the depth direction have the proportional relation, which implies that as the transmission voltage is made higher, the diagnostic depth can be made deeper. Thus, it is desired to make the transmission voltage as high as possible. In view of those facts, the many trials of lowering the surface temperature of the ultrasonic probe have been tried in recent years. The structure shown in FIG. 8 is one of them and is configured to dissipate the heat from the grounded electric terminal 23 taken out from the ground electrode 2 of the piezoelectric element 21.    Patent Document 1: Japanese Patent Application publication (JP-A-Heisei, 5-244690) (FIG. 1)
However, the heat radiation in the configuration of the conventional ultrasonic probe is the heat radiation from a part of the grounded electric terminal 23 of the piezoelectric element 21, and there is a problem that it is not always sufficient.